1.) Field of the Invention
The present invention relates to a display device, a lighting device, and a projector, and more particularly, to a display device and a lighting device used in a projector.
2.) Description of the Related Art
As flat panel displays (FPD), there have been known displays with a two-dimensional array of light emitting diodes (LED) that are solid-state light emitting elements used as a back light (see I. Hiyama et al., LN-3: “Four-Primary Color 15-in. XGA TFT-LCD with Wide Color Gamut”, EURODISPLAY2002, PP 827–830). In such an FPD, one frame is divided into a pair of subframes, and displays of colored lights G1 light and G2 light that are approximate to a green light (hereinafter, “G light”) are allocated to the subframes, respectively, and displays of a red light (hereinafter, “R light”) and a blue light (hereinafter, “B light”) are also allocated thereto, respectively. Therefore, a first subframe displays images of the R light and the G1 light, and a second subframe displays images of the G2 light and the B light. Consequently, an image can be expressed with four colored lights, which allows a color reproduction range to be widened.
The LED has advantages such that it has generally longer life as compared with that of an extra-high pressure mercury lamp or the like and light conversion efficiency is high. Therefore, the LED is increasingly used as a light source of a lighting device. The LED as a single unit has an amount of light emission smaller as compared with that of the extra-high pressure mercury lamp or the like. A light source of a projector requires a comparatively larger amount of light. Therefore, some structures to increase the amount of light have been proposed to use the LED as the light source of the projector. For example, Japanese Patent Application Laid Open No. 2001-42431 discloses a structure of illuminating a liquid-crystal-type light valve as an example of a spatial light modulator of the projector using a light source device that includes many LEDs having different light emission wavelengths. In the light source device, luminous fluxes emitted from a pair of LEDs having slightly different light emission wavelengths are combined in a dichroic mirror, that is, colors are combined to increase the intensity of a colored light having a particular wavelength range, and the amount of light is thereby increased.
However, in the FPD, two-color illumination and display are performed simultaneously, and therefore, a color filter is essential, which causes the structure of the LCDs, i.e., an image display portion to be complicated and the loss of light amount at the filter to occur. Furthermore, in the FPD, one frame is divided into a pair of subframes, and it is therefore required to drive the LCD or the like at double speed, which causes generation of control signals corresponding to respective colors to become extremely complicated. Moreover, lights are emitted from only two LEDs out of four LEDs corresponding to four colors. As a result, a long dead time occurs in the LEDs, that is, the LEDs are not made effective use of. It can be considered that the four LEDs corresponding to the four colors are simultaneously lit by using an appropriate filter. However, a difference in intensity between the G1 light and the G2 light in the divided portions may become extremely large depending on display images. Therefore, when lighting is switched between the G1 light and the G2 light, the intensity of a pixel for adjacent R light or B light may unstably fluctuate due to influence of the switching between the subframes.
Generally, in the dichroic mirror for color combination, a wavelength, i.e., a cutoff frequency in the vicinity of a wavelength in which transmittance characteristic or reflectance characteristic is largely changed, is different from another cutoff frequency between a p-polarized light and an s-polarized light. Therefore, it is required to set a light emission central wavelength (hereinafter, “peak wavelength”) of a pair of LEDs in both sides of the outside of a wavelength range between a pair of cutoff frequencies corresponding to both the polarized lights. Such a difference between the peak wavelengths sometimes reaches about 50 nanometers (nm). Therefore, in the light source device disclosed in the publication, peak wavelengths of a pair of luminous fluxes emitted from the pair of LEDs cannot be made closer to a predetermined value or more caused by optical characteristics of the dichroic mirror. As a result, the difference between the peak wavelengths of the pair of luminous fluxes becomes larger, which results in occurrence of a problem such that color purity of a particular color obtained after wave combination is reduced. If a pair of luminous fluxes having close peak wavelengths is made to enter the dichroic mirror, a part of the fluxes is reflected by the dichroic mirror although all the fluxes are desired to pass through it, which results in the loss of light amount, or a part of the fluxes passes through the dichroic mirror although all the fluxes are desired to be reflected thereby, which results in the loss of light amount. Therefore, it is difficult to obtain high-intensity illumination light with high color purity based on the conventional structure.
It is further considered that a plurality of LEDs is arranged in an array as a structure for increasing a light amount from the light source using LED. This arrangement allows the light amount to be increased in proportion to the number of LEDs. In an optical system including the light source and the spatial light modulator of the projector, spatial extent where a luminous flux capable of being effectively handled is present can be expressed as a product of area and solid angle (Etendue, Geometrical Extent). The product of area and solid angle is stored in the optical system. Therefore, if the spatial extent of the light source becomes wider, the spatial extent, where the luminous flux incident on the spatial light modulator is present, increases. However, an angle of the luminous flux that can be captured by the spatial light modulator is limited, which makes it difficult to effectively use the luminous flux from the light source. When a plurality of LEDs is arranged in an array to increase the light amount, the area (spatial extent) of the light source becomes also wider. Therefore, in the projector, even if the LEDs are simply arranged in an array to increase the light amount, it is difficult to effectively use all the luminous fluxes from the light source because the etendue is stored. As a result, the light amount cannot be increased.
An object of the present invention is to provide a display device capable of displaying a high-luminance image with a wide color reproduction range under simple control, a lighting device capable of supplying high-intensity illumination light with high color purity, and a projector including the lighting device.